The present invention relates to non-slip transmissions particularly useful as continuously-variable transmissions, and also to transmission members included therein.
The term “continuously variable transmission” (CVT) is used herein in its normal and accepted sense, namely as a transmission in which the ratio of the rotational speeds of two shafts can be varied continuously, without break or interruption, within a given range, to provide an infinite number of possible ratios”. A “continuously variable transmission” (typically using rubber belts and pulleys) is to be sharply distinguished from a “step-variable transmission” (typically including gears), which provides a finite number of possible ratios according to the steps (e.g. gear teeth), rather than an infinite number of possible ratios without break or interruption.
A main problem in many types of existing continuously-variable transmissions (CVTs) is slippage between the contacted surfaces. Slippage causes rapid wear of the contact surfaces, together with energy losses and low efficiency. These problems become more serious as the transmitted torque increases. For this reason, continuously-variable transmissions currently find little use in machines requiring high torque transmission, such as medium and heavy vehicles and industrial machinery.
The continuously-variable transmissions used at present are generally based on high-pressure contact between two smooth surfaces having a high coefficient of friction, rather than on contact between metallic bodies such as gear wheels which cannot slip and which have a low coefficient of friction. The commonest examples of the presently-used systems are those based on a V-belt made of rubber. The high coefficient of friction and the high pressure between the surfaces are intended to prevent slippage, but even this is insufficient in certain cases. Such systems may be adequate for the transmission of small torques, but are generally considered to be uneconomical and inefficient when applied to the transmission of high torques.
Transmission systems based on gears include a driving member and a driven member, engaging each other by matching sets of projections and depressions (gear teeth) that force one member to move along with the other member without slippage. The transmission ratio between the driving member and the driven member in these gears is constant.
As the transmission ratio of such gears is determined by the ratio between the number of cogs or teeth (projections) on each member, they cannot be used for producing a variable transmission ratio: If the diameter of one member is changed without changing the number of teeth, then the pitch between the teeth will not match the other member; and if the diameter is changed while preserving the pitch, then the number of teeth around the member will, at times, be fractional—making it impossible to continuously engage with the other member.
Several patents have issued on methods to create a non-slip continuously variable transmission (CVT), including the following:
U.S. Pat. No. 1,650,449 (Jaeger) and U.S. Pat. No. 4,952,196 (Chilcote) disclose a CVT in which two wheels change their overall diameters so that a fixed length chain is suspended around both. For many wheel diameters in this method, the circumference of the wheel is not an integer number of teeth; therefore, a link of the chain will meet the circumference of the wheel out of phase. Special cumbersome means are therefore necessary to overcome this problem, if possible at all.
U.S. Pat. No. 1,601,662 (Abbott) discloses a CVT that addresses the above problems by a conical structure in which coupling elements in one member adjust their position to match the topography of the other member in each and every diameter. This method requires the coupling elements to converge to their operational position through a cumbersome back and forth trajectory in which they slide on and collide with other parts of the topography in a non-matching angles of contact, creating increasing friction and bending moments.
U.S. Pat. No. 6,055,880 (Gogovitza) discloses another conical approach of a CVT that ensures positive engagement at any transmission ratio, and addresses the problem of U.S. Pat. No. 1,601,662 by bringing each coupling element directly to its operational position. However, the conical structure of the system of this patent creates a non-uniform pitch and a non-uniform speed along the line of contact between the coupling element and the topography, resulting in the creation of undue differential stresses, bending moments, and slippage between the coupled elements. In practice, this system implies a small number of small lines of contact—thus a limited transmission of moments.